Conventionally, various measures are adopted to provide a radio system having an excellent phase noise characteristic. An example of this conventional radio system having an excellent phase noise characteristic is described in Patent Document 1. This radio system is provided with a local noise canceller shown in FIG. 14 to improve its phase noise characteristic.
The operation of this local noise canceller will be explained with reference to FIG. 14 and FIG. 15. FIG. 15 shows characteristic diagrams illustrating frequency characteristics of the respective components of the local noise canceller shown in FIG. 14.
As shown in FIG. 15A, suppose that an input signal consists of a modulated IF signal (BST-OFDM) multiplexed with pilot carrier (PILOT) and input phase noise (part expressed by bold hatching) is superimposed thereon.
Here, when it is assumed that the frequency of the input pilot carrier is fPLT, frequency of the input signal is fsig and input phase noise is θ(t), input phase noise θ(t) is superimposed on fPLT and fsig, and therefore they are expressed as follows:fPLT∠θ(t)fsig∠θ(t)
Input signal A is distributed by distributor 50 and one portion is output to a pilot branch and the other to a signal branch. In the pilot branch, the one signal distributed by distributor 50 is band-limited by band pass filter 51, only a pilot carrier component thereof is allowed to pass and extracted and further subjected to limiter amplification by limiter amplifier 52.
At this time, the IF signal component is removed from the frequency characteristics of output signal B from band pass filter 51 and output signal C from limiter amplifier 52 as shown in FIG. 15B, 15C, and only the pilot carrier component and input phase noise θ(t) superimposed thereon remain.
At this time, a delay is produced at band pass filter 51 and if this delay time is assumed to be τBPF1, input phase noise θ(t−τBPF1) delayed by τBPF1 is superimposed on input pilot carrier frequency fPLT, and therefore fPLT is expressed as follows:fPLT∠θ(t−τBPF1)
On the other hand, in the signal branch, local oscillation signal D is output from local oscillator 60. Here, the frequency characteristic of local oscillation signal D output from local oscillator 60 consists of a signal of local oscillation frequency (LO) and phase noise originating in the system which is superimposed thereon as shown in FIG. 15D.
Here, if it is assumed that the local oscillation signal frequency in the system is fLO and the local oscillation signal phase noise in the system is φ(t), local oscillation signal phase noise φ(t) in the system is superimposed on the local oscillation signal frequency fLO in the system, and therefore fLO is expressed as follows:fLO∠φ(t)
In the signal branch, the frequency of the signal output from distributor 50 is converted by frequency converter 61 using local oscillation signal D from local oscillator 60 and signal E is output.
Here, the frequency characteristic of signal E output from frequency converter 61 includes a sum component and difference component between input signal A and local oscillation signal D as shown in FIG. 15E. Thus, relationships between the respective signal components included in signal E and phase noise to be superimposed thereon are expressed as follows:fPLT−fLO∠θ(t)−φ(t)fsig−fLO∠θ(t)−φ(t)fPLT+fLO∠θ(t)+φ(t)fsig+fLO∠θ(t)+φ(t)
Frequency-converted signal E is band-limited by band pass filter 62 so that only the difference component passes, and therefore it is output from band pass filter 62 as signal F and the frequency characteristic of signal F is deprived of the sum component in E and only the difference component exists as shown in FIG. 15F.
At this time, a delay is produced at band pass filter 62 and if this delay time is assumed to be τBPF2, delay τBPF2 is produced in the phase noise superimposed on the extracted difference component and relationships between the respective signal components included in signal F and phase noise to be superimposed thereon are expressed as follows:fPLT−fLO∠θ(t−τBPF2)−φ(t−τBPF2)fsig−fLO∠θ(t−τBPF2)−φ(t−τBPF2)
A delay is then added to signal F by delay compensator 63 so as to be equivalent to the delay time at band pass filter 51 in the pilot branch and the signal with the delay is output as signal G.
Here, when it is assumed that the delay time of band pass filter 62 with respect to delay time τBPF1 of band pass filter 51 is τBPF2 and the delay time at delay compensator 63 is Δt, delay compensator 63 adds delay Δt to signal F in such a way thatτBPF1=τBPF2+Δt to equalize the delay time difference between the signal branch and pilot branch.
As a result, the frequency characteristic of signal G does not change as shown in FIG. 15G and relationships between the respective signal components included in signal G and phase noise to be superimposed thereon consist of phase noise plus delay Δt as follows:fPLT−fLO∠θ(t−τBPF2−Δt)−φ(t−BPF2−Δt)fsig−fLO∠θ(t−τBPF2−Δt)−φ(t−τBPF2−Δt)
Signal G in the signal branch and signal C in the pilot branch output from above described limiter amplifier 52 are frequency-converted by frequency converter 70 and output as signal H.
Here, the frequency characteristic of signal H output from frequency converter 70 includes the sum component and difference component between signal G and signal C as shown in FIG. 15H. Therefore, relationships between the respective signal components included in signal H and phase noise to be superimposed thereon are expressed as follows:fPLT−(fPLT−fLO)∠θ(t−τBPF1)−{θ(t−τBPF2−Δt)−φ(t−τBPF2−Δt)}fPLT−(fsig−fLO)∠θ(t−τBPF1)−{θ(t−τBPF2Δt)−φ(t−τBPF2−Δt)}fPLT+(fPLT−fLO)∠θ(t−τBPF1)+{θ(t−τBPF2−Δt)−φ(t−τBPF2−Δt)}fPLT+(fsig−fLO)∠θ(t−τBPF1)+{θ(t−τBPF2−Δt)−φ(t−τBPF2−Δt)}
Here, as shown above, delay compensator 63 adds delay Δt in such a way that:τBPF1−τBPF2−Δt to equalize the delay time difference between the signal branch and pilot branch, and therefore the expression can be organized as follows:fLO∠θ(t−τBPF2−Δt)fLO−(fsig−fPLT)∠φ(t−τBPF2−Δt)2×fPLT−fLO∠2×θ(t−τBPF1)−φ(t−τBPF2−Δt)fPLT+(fsig−fLO)∠2×θ(t−τBPF1)−φ(t−τBPF2−Δt)
Here, if attention is focused on the difference component, the frequency of the output signal component is frequency (fLO) of the local oscillation signal in the system irrespective of the frequency of the input signal, that is, constant. Furthermore, the side band of the signal when focused on the pilot carrier is reversed between the input and output.
Furthermore, input phase noise θ(x) is canceled and the phase noise of the output signal becomes phase noise φ(x) of the local oscillation signal in the system instead. That is, it is understandable that when phase noise φ(x) of the local oscillation signal in the system is small enough, the phase noise of the input signal is sufficiently reduced and output.
Thus, signal H frequency-converted by frequency converter 70 is band-limited by band pass filter 71 so that only the difference component passes and signal I is output so that only the signal component passes, and the frequency characteristic of signal I is deprived of the sum component in H and the pilot carrier component in the difference component as shown in FIG. 15I, includes only the signal component of the difference component and the relationship between the signal component included in signal I and phase noise to be superimposed thereon is as follows:fLO−(fsig−fPLT)∠φ(t−τBPF2−Δt)
Based on the principles of frequency synchronization and noise cancellation by the above described local noise canceller, even if a frequency deviation is produced on an input signal, for example, it is possible to obtain an output signal at a frequency following the local oscillation frequency having a high degree of stability with high frequency accuracy generated by local oscillator 60 and thereby eliminate the frequency deviation of the input signal.
Furthermore, with phase noise θ(x) superimposed on the input signal canceled, the phase noise of the output signal becomes only phase noise φ(x) of the local oscillation signal in the system instead, and therefore if phase noise φ(x) of the local oscillation signal in the system is small enough, the phase noise of the input signal is sufficiently reduced and output.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-152158